Free-piston internal combustion engines are linear engines in which the need for a crankshaft system is eliminated and the power piston (or pistons) and associated components have a purely linear motion. FIG. 1 illustrates the configuration of a dual-piston free-piston engine system known from the prior art. The engine includes two opposing combustion cylinders, each being similar to those known from conventional two-stroke cycle crankshaft engines. The two combustion cylinder pistons are rigidly connected and form a piston assembly, which is the only significant moving component. The piston assembly can move linearly, the outer limits of the motion being restricted by the combustion cylinders. Two-stroke cycle operation in each cylinder maintains a reciprocating motion of the piston assembly. A power stroke is performed alternately by each of the two pistons, such that a power stroke in one cylinder drives the compression stroke in the other cylinder. This eliminates the need for a rebound device, used in a single-piston free-piston engine for storing energy generated in the power stroke for compressing the next cylinder charge. Incorporated into the system is a linear electric machine, with a translator (usually comprising permanent magnets) fixed to the piston assembly and a stator (comprising coils) fixed to the engine housing, allowing conversion of additional surplus energy into electric energy.
The potential advantages of free-piston engine systems compared to conventional, crankshaft engines are numerous. The simplicity of the engine and reduced number of parts compared to a conventional engine reduce frictional losses and wear, as well as engine size, weight, and manufacturing costs. The absence of bearings carrying high loads, such as those found in the crank system in conventional engines, allows operation with high in-cylinder pressures, benefiting fuel efficiency. Moreover, the compression ratio in a free-piston engine is variable, which allows extensive operational optimisation for different operating conditions (such as load level), as well as for different fuels.
Examples of publications describing free-piston engine systems include U.S. Pat. No. 2,900,592, U.S. Pat. No. 4,924,956, U.S. Pat. No. 5,002,020, U.S. Pat. No. 6,199,519, and U.S. Pat. No. 6,541,875. An overview of this technology was presented by Mikalsen and Roskilly in Applied Thermal Engineering, 2007; 27:2339-2352.
There are two main challenges associated with prior art free-piston engine systems which have prevented their commercial success.
First, the free-piston engine is, in its standard configuration, restricted to the two-stroke operating principle, since a power stroke is required every reciprocation cycle to maintain engine operation. In a conventional crankshaft engine, the energy stored in the crank system and flywheel can drive the piston during the gas exchange strokes of a four-stroke cycle, giving the engine designer a choice between two-stroke and four-stroke operation. In the free-piston engine, no such energy storage exists. It is well known that small to medium size two-stroke cycle engines suffer from poor fuel efficiency and high exhaust gas emissions compared to four-stroke engines, and it is therefore currently used only in a limited number of applications. The main reason for the poorer performance is the inefficient gas exchange process in two-stroke engines. Scavenging of the cylinder is achieved by the simultaneous opening of inlet and exhaust ports while the piston is in the lower part of the cylinder (around bottom dead centre). Achieving efficient scavenging, in which all combustion products are displaced by fresh charge, is extremely challenging, and typically only a replacement of 60-80 percent of the combustion products from a previous cycle can be achieved. Furthermore, since the inlet ports and exhaust ports (or valves) necessarily must be open simultaneously, there will be some flow of inlet charge directly to the exhaust system (known as short-circuiting). This has significant adverse effects on both fuel efficiency and exhaust gas emissions levels.
Some alternative configurations have been proposed to allow four-stroke operation in free-piston-type engines. One example was described in U.S. Pat. No. 7,258,086, which used a four cylinder configuration in which one of the cylinders at any time performed a power stroke. Mechanical linkages were then used to drive the non-power strokes in the other cylinders. However, the additional complexity in these systems removes several of the key advantages of the free-piston engine concept, including compactness, a low number of moving components, and no load-carrying bearings or linkages.
The second fundamental challenge associated with the free-piston engine concept is the control of the piston motion. In a conventional crankshaft engine, the high inertia of the crank system and flywheel stabilises engine operation, in particular during rapid load changes or cycle-to-cycle variations in the combustion process. In the free-piston engine, these will have a significantly larger effect on engine operation. Since the piston motion in a free-piston engine is not restricted by a crankshaft, a sufficiently high kinetic energy of the piston assembly, for example due to a rapid load decrease, may lead to mechanical contact between the piston and cylinder head, which may be catastrophic for the engine. Conversely, a reduction in kinetic energy, for example due to a rapid load increase, may lead to a failure to reach sufficient compression of the in-cylinder charge and the engine stalling.